This invention relates to catalyst carriers based on titania and particularly to carriers that can be used to make catalyst particles that are particularly useful in hydrocarbon conversion processes.
Titanium dioxide, (titania), is commonly obtained from raw materials such as ilmenite ore or "titanium slag" (residue after extraction of titanium), by either the chloride process or the sulfate process. In the chloride process the raw material is reacted with chlorine to yield titanium tetrachloride which is then burned to give titanium dioxide. The product can be obtained as very uniform excellent quality rutile or a quite adequate anatase or mixtures of these phases and with a BET surface area of anything from 8 to about 60 m.sup.2 /gm.
The alternative route involves digestion of the titania-containing raw material in sulfuric acid and separating from the mixture a solution of a titanium sulfate which is then hydrolyzed to give titanic acid with some associated sulfuric acid. Yet another process involves the acid hydrolysis of titanium tetrachloride to give titanic acid with associated residual acid. The product obtained by such acid hydrolytic processes is commonly referred to as "hydrate pulp". Calcination of this hydrate pulp can yield good quality anatase or rutile forms with surface areas of from 1 to 16 m.sup.2 /gm. This hydrate pulp has a surface area that is typically more than about 200 m.sup.2 /gm.
Titania based carriers are widely used to support catalyst compositions that are to be exposed to elevated temperatures in use The carriers can have "high", "low" or "intermediate" surface areas depending on the application. In the context of such applications "low" means a surface area of below about 10 m.sup.2 /g for example less than about 8 m.sup.2 /g; "intermediate" means a surface area of about 10 to 100 m.sup.2 /g, for example from about 15 to about 95 m.sup.2 /g; and "high" means over about 100 m.sup.2 /g.
The specific application to which such carriers can be put is very wide including the catalytic formation of amines as taught for example in U.S. Pat. No. 5,225,600; diesel engine exhaust gas purification as disclosed in U.S. Pat. No. 5,208,203; decomposition of organic peroxides to from alcohols using the process of U.S. Pat. No. 4,547,598; removal of peroxide contaminants from alcohol product streams according to the process of U.S. Pat. No. 5,185,480; and in the Fischer-Tropsch process as set out in U.S. Pat. No. 5,169,821. Many of these applications prefer the use of high, (often very high), surface area catalyst supports. It is found however that such supports have little mechanical strength or attrition resistance.
The present invention concerns titania supports for example those intended for use in applications where attrition resistance is very important. These are applications in which the titania is in the form of pellets or extruded shaped particles designed to have a large geometrical surface area, an intermediate or even low BET surface area and, importantly, excellent attrition resistance enabling the catalyst to be used in situations in which the carrier particles can be expected to be subject to abrasive contact with adjacent particles and to substantial compressive forces under the conditions of use, that is, applications requiring particles with high crush strength.
The primary applications for such carriers is therefore in the area commonly referred to as "low" and "intermediate" surface area catalysts though they may also be useful in high surface area catalyst applications.
There is however a problem with obtaining a titania-based carrier that has adequate strength to withstand an environment in which a significant amount of crush strength is required. In a tower packed with extruded catalyst bearing carrier particles those at the bottom of the tower must withstand significant compressive forces.
Carrier materials are commonly produced by mixing a titania powder with a temporary binder formulation until an extrudable paste is formed, then forming the paste into the desired shape, drying the shape and firing to burn out the temporary binder and to convert the titania to a solid stable material. The titania carrier can be obtained in the shape of pellets, (as a result of extruding a continuous rod and then cutting the rod into pellets of the desired size), or it may be in the form of a large honeycomb monoliths, or it may be in individual relatively small, ring-based shaped structures of any desired configuration such as "wagon wheels" or any other extruded shapes with constant cross-sections such as for example multi-lobed structures and small honeycombs.
The higher the temperature to which the titania is fired, the more the structure is sintered and the stronger and more attrition-resistant it becomes. Unfortunately this is also accompanied by a reduction in surface area. As the temperature of sintering rises titania changes phase from the amorphous phase to the anatase form. Then above about 800.degree. C. it begins to transform to the rutile form. The actual transformation temperature may be affected by the presence of impurities but it is generally complete by about 950.degree. C. Increased levels of sintering are accompanied by a reduction in the surface area until a carrier that started off as being a "high" surface area carrier comes to be classed as "low". Indeed heating for too long a period at too high a temperature can result in the material losing any practical utility as a carrier. A useful carrier must also have sufficient porosity to carry catalytic amounts of a metal.
For many applications in which a very high surface area is preferred, the compressive strength is less important. If for example the carrier is in the form of a ceramic honeycomb monolith it is subjected to little in the way of compressive forces or attrition. For such applications the very high BET surface areas available with a lightly fired anatase support is quite suitable.
There is however also a need for a carrier with an adequately high surface area while maintaining a high level of attrition resistance and strength making it adaptable for very demanding catalytic applications.
The novel process of the present invention yields a carrier having properties that are not dependent only on the primary particle size of the starting material but also on the amount of volatilizable material it contains.